A woman helps adjust a mask for her friend outside an amusement park on a hazy day in Beijing, where air pollution levels were dangerously high and people were warned to stay indoors.

Photo: Alexander F. Yuan, Associated Press

A woman helps adjust a mask for her friend outside an amusement...

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A man flies a kite near electricity pylons on a hazy day in Beijing Saturday, Jan. 12, 2013. Air pollution levels in China's notoriously dirty capital were at dangerous levels Saturday, with cloudy skies blocking out visibility and warnings issued for people to remain indoors. (AP Photo/Alexander F. Yuan)

Photo: Alexander F. Yuan, Associated Press

A man flies a kite near electricity pylons on a hazy day in Beijing...

One day more than two years ago, an air-quality monitoring device atop the U.S. Embassy in Beijing recorded data so horrifying that someone in the embassy called the level of pollution "Crazy Bad" in an infamous Twitter post.

That day the Air Quality Index, which uses standards set by the Environmental Protection Agency, had crept above 500, which was supposed to be the top of the scale.

So what phrase is appropriate to describe Saturday's jaw-dropping reading of 755 at 8 p.m., when all of Beijing looked like an airport smokers' lounge? Though an embassy spokesman said he did not immediately have comparative data, Beijing residents who follow its Twitter feed said the Saturday numbers appeared to be the highest recorded since the embassy began its monitoring system in 2008.

According to the EPA, levels between 301 and 500 are "Hazardous," meaning people should avoid all outdoor activity. The World Health Organization has standards that judge a score above 500 to be more than 20 times the level of particulate matter in the air deemed safe.

By comparison, the air quality index in New York City was 19 at 6 a.m. Saturday.

It was unclear exactly what was responsible for the rise in levels of particulate matter, beyond the factors that regularly sully the air here. Factories operating in neighboring Hebei province ring this city of more than 20 million. The number of cars on Beijing's streets has been multiplying at an astounding rate. And Beijing sits on a plain flanked by hills and escarpments that can trap pollution.

The existence of the embassy's monitor and the@BeijingAir Twitter feed have been a diplomatic sore point for Chinese officials. In July 2009, a Chinese Foreign Ministry official, Wang Shu'ai, told American diplomats to halt the Twitter feed, saying the data "is not only confusing but also insulting," according to a State Department cable obtained by WikiLeaks. Wang said the embassy's data could lead to "social consequences."